The present invention relates to a liquid crystal display device; and, more particularly, the invention relates to a liquid crystal display device of the active matrix type, such as a thin film transistor (TFT) type, and a method of manufacture of the same.
A liquid crystal display device, particularly an in-plane-switching (hereinafter referred to as IPS) type of liquid crystal display device, in which a lateral electric field is applied to the liquid crystal gap between upper and lower substrates, is recognized as a display type that is capable of satisfying the demand for a high image quality, and various improvements have been made to the IPS type of liquid crystal display device for the purpose of further improving the image quality thereof.
One example of the IPS mode of liquid crystal display device is a type which switches a liquid crystal by means of a lateral electric field that is generated between two layers of metal electrodes having an insulating film interposed therebetween. This structure has the disadvantage that it is difficult to increase the pixel aperture ratio and the efficiency of utilization of light is low, as compared with a TN type (twisted nematic scheme) of display device. Since the luminance of the backlight must be increased to compensate for this disadvantage, it is difficult to lower the total power consumption of an LCD module to the extent required for notebook types of personal computers or portable terminals.
The IFS type of liquid crystal display device has a problem in that a higher aperture ratio, i.e., a higher transmissivity, is needed because of its low aperture ratio. An investigation made by the present inventor has shown that it is desirable to completely shield gate lines serving as electric-field generating sources, except for the pixel electrodes and common electrodes which constitute comb teeth-shaped electrodes, from the point of view of achieving a reduction in the non-uniformity of the image quality due to penetration of electric fields and the prevention of a decrease in contrast ratio due to light leakage.
Furthermore, it has been found that, in the IPS type of liquid crystal display device, the capacitance between the comb teeth-shaped electrodes, i.e., the liquid crystal capacitance, is small compared to that in the TN liquid crystal display device in which plane-shaped electrodes are opposed to each other with a liquid crystal layer interposed therebetween, and a large charge-holding capacitance is needed to prevent deterioration of the display quality due to potential variations, resulting in the problem that, if the charge-holding capacitance is increased for this reason, the aperture ratio decreases.
One object of the present invention is to solve the above-described problems and to provide a bright liquid crystal display device, in which shielding of the gate lines, which prevents malfunctions in the image display, is improved, the display quality is improved, and the aperture ratio is improved. Other advantages of the invention and other problems to be solved by the invention will become apparent from the following description.
Main aspects of the invention for solving such problems are as follows.
(1) A lateral electric field type of liquid crystal display device (active matrix type liquid crystal display device) includes a first substrate and a second substrate disposed in opposition to each other with a liquid crystal layer interposed therebetween, a plurality of gate lines formed on the first substrate, a plurality of drain lines formed to intersect the plurality of gate lines in a matrix array, thin film transistors respectively formed at intersections of the gate lines and the drain lines, and pixels each provided in an area surrounded by a pair of gate lines adjacent to one another and a pair of drain lines adjacent to one another. (a) The first substrate includes, in each of the pixels, a semiconductor layer formed on the first substrate, a first insulating film formed on the semiconductor layer, a gate line formed on the first insulating film, a second insulating film formed on the gate line, a drain line formed on the second insulating film, a third insulating film formed on the drain line, and a common electrode or a common electrode line formed on the third insulating film. (b) The semiconductor layer is connected to a corresponding one of the thin film transistors near which one of the pair of the gate lines arranged adjacent to one another is formed, and it has an area which is formed under another of the pair of the gate lines arranged adjacent to one another with the first insulating film interposed therebetween.
(2) In the liquid crystal display device as described in paragraph (1), the semiconductor layer is formed under the other of the pair of the adjacent gate lines with the first insulating film interposed therebetween, and an electrode is formed over the other of the pair with at least the second insulating film interposed therebetween.
(3) In the liquid crystal display device as described in paragraph (2), the electrode is formed as a metal electrode.
(4) In the liquid crystal display device as described in paragraphs (2) or (3), a shield electrode for leakage electric fields from the another gate line is formed by the semiconductor layer formed under the another of the pair of the gate lines adjacent to one another with the first insulating film interposed therebetween and the electrode formed over the another gate line with at least the second insulating film interposed therebetween.
(5) In the liquid crystal display device as described in paragraphs (2) to (4), at least a side portion of the another of the pair of the gate lines adjacent to one another is, as seen in plan view, covered with the common electrode and the electrode formed with at least the second insulating film interposed between the electrode and the another of the pair.
(6) In the liquid crystal display device as described in any of paragraphs (1) to (5), the third insulating film is an organic film.
(7) In the liquid crystal display device as described in any of paragraphs (1) to (6), the common electrode formed on the third insulating film is a transparent electrode.
(8) In the liquid crystal display device as described in paragraphs (4), the insulating film interposed between the semiconductor layer and the electrode which constitutes the shield electrode is formed by stacking a gate insulating film of the thin film transistor and an interlayer insulating film which separates the gate line and the drain line.
(9) A lateral electric field type of liquid crystal display device (active matrix type liquid crystal display device) includes a first substrate and a second substrate disposed in opposition to each other with a liquid crystal layer interposed therebetween, a plurality of gate lines formed on the first substrate, a plurality of charge-holding lines, a plurality of drain lines formed to intersect the plurality of gate lines in matrix form, thin film transistors respectively formed at intersections of the gate lines and the drain lines, and pixels each provided in an area surrounded by a pair of gate lines adjacent to one another and a pair of drain lines adjacent to one another. (a) The first substrate includes, in each of the pixels, a semiconductor layer formed on the first substrate, a first insulating film formed on the semiconductor layer, a gate line formed on the first insulating film, a second insulating film formed on the gate line, a drain line formed on the second insulating film, a third insulating film formed on the drain line, and a common electrode formed on the third insulating film. (b) The charge-holding lines, each of which extends in an extending direction of a corresponding one of the drain lines, is provided under the corresponding drain line, and the common electrode which extends in an extending direction of each of the drain lines is provided over each of the drain lines.
(10) In the liquid crystal display device as described in paragraph (9), in each of the pixels, the semiconductor layer extends under the drain line and has an area which extends into a portion under a corresponding one of the charge-holding lines extending in the extending directions of the respective drain lines and forms a charge-holding capacitance.
(11) In the liquid crystal display device as described in paragraph (10), the semiconductor layer is connected to a corresponding one of the thin film transistors near which one of the pair of the gate lines adjacent to one another is formed, and has an area which is formed under another of the pair of the gate lines adjacent to one another with the first insulating film interposed therebetween, whereby the semiconductor layer forms a shield electrode for leakage electric fields with respect to one of the pair of the gate lines adjacent to one another, and forms a charge-holding capacitance with respect to the corresponding one of the charge-holding lines extending under the respective drain lines.
(12) In the liquid crystal display device as described in any of paragraphs (9) to (11), the charge-holding lines and the common lines are connected to one another outside a display area and are given the same potential.
(13) In the liquid crystal display device as described in any of paragraphs (1) to (12), the semiconductor layer connected to the thin film transistor is made of polysilicon (poly crystalline silicon film).
Further aspects of the invention will become apparent from the following description of various embodiments of the invention.